DE69321073T2 - Capture a variety of items - Google Patents

Abstract

An identification system comprises an interrogator and a plurality of transponders. The interrogator includes a transmitter which transmits an interrogation signal and an inhibiting signal to the transponders, and a receiver for receiving response signals from the transponders. The interrogator also includes processor means for identifying the transponders from data in the received response signals and means for disabling transponders. The transponders which are disabled remain disabled whilst they continue to receive the inhibiting signal. Each transponder comprises receiving means, a code generator, and transmitter means connected to the code generator. On receipt of the transmitted interrogation signal the transponder transmits a response signal containing data which identifies it.

Description

Background of the invention

The invention relates to an identification system comprising an interrogation device and a plurality of response devices (transponders).

South African Patent Application No. 92/0039 describes an identification system comprising an interrogator and a number of individual responders that can be attached to or associated with articles to be identified. The articles to be identified can be, for example, goods in a supermarket or warehouse.

The invention is based on the object of increasing the probability of identifying a responder in a system of the aforementioned type.

State of the art

EP 494114 A discloses an interrogator/responder system in which an interrogator transmits an interrogation signal to a plurality of responders present in the interrogation area. An example of the identification system comprises an interrogator or reader transmitting interrogation signals with a power of about 15 W and a frequency of about 915 MHz to a number of passive responders. The responders derive power from the energy in the interrogation signal and modulate a portion of the energy received from the interrogator with an identification code to generate a response signal which is transmitted back to the interrogator.

GB 2152335 A discloses an inventory control device using tags that respond to a specific pair of frequencies and simultaneously respond to a third frequency. An interrogator steps through a programmed set of all relevant frequency pairs to interrogate a plurality of tags, with the interrogator's receiver tuned to the third frequency. The amplitude of the third frequency is a function of distance, receiver antenna directivity and the number of tags present.

WO 8200910 A discloses an ultrasonic communication and security system. A plurality of intrusion sensors can be interrogated by a central data unit using signals transmitted at two different frequencies. The frequencies are sufficiently separated so that the zero points are spatially separated due to multipath transmission.

Summary of the invention

According to a first aspect of the invention there is provided an identification system comprising an interrogator and a plurality of responders, the interrogator comprising transmitter means for transmitting an interrogation signal to the responders, receiver means for receiving response signals from the responders and processing means for identifying the responders based on the data in the response signals, each responder comprising receiving means, a code generator and transmitting means connected to the code generator so that on receipt of the transmitted interrogation signal the responder transmits a response signal containing data identifying the responder, the interrogator being capable of disabling each responder, characterized in that the interrogator transmits at least two intermittent interrogation signals, there being an interval between successive interrogation signals which is shorter than a minimum period within which responders which have been disabled can automatically reset themselves.

The at least two interrogation signals can each have different frequencies which are selected so that they fall within the reception bandwidth of the receiving devices of the response devices.

Preferably, the at least two interrogation signals are signals with a relatively narrow bandwidth, the receiving devices of each response device having a relatively wide reception bandwidth into which the different frequencies of the at least two interrogation signals fall, so that the response device responds to each individual or multiple interrogation signals.

Preferably, each interrogation signal is modulated with data, wherein the data modulation bandwidth of each interrogation signal is less than the distance between the respective different frequencies of the interrogation signals.

The transmitting means of the responder may comprise an antenna and means for modulating the reflectivity of the antenna so that the responder's response signal comprises one or more interrogation signal carriers modulated with the data identifying the responder.

Preferably, the transmitter device of the interrogation device comprises at least two transmitting antennas spaced apart from one another and the receiver device comprises at least two receiving antennas spaced apart from one another.

The transmitter device and the receiver device can comprise at least two spaced-apart antenna units, each antenna unit comprising a transmitting antenna and an adjacent receiving antenna.

Each antenna may comprise a circuit arrangement designed to operate at a frequency of between 800 MHz and 1 GHz.

Preferably, at least two of the respective transmitting and receiving antennas are polarized differently from one another.

The transmitter means and the receiver means of the interrogator may be mounted on or adjacent to a structure that defines an interrogation area through which the responders to be identified can be passed.

In a preferred embodiment, the transmitter means and the receiver means of the interrogator are supported by a frame defining a passageway through which transport of articles to which the respective responders are attached can take place.

The respective different frequencies of the at least two interrogation signals are preferably selected such that there are no overlapping, dead spots in the electrical fields of the interrogation signals within a predetermined distance of the transmitter device of the interrogation device.

The system may include processing means for recording data received from each identified responder and for relating the received data to stored data corresponding to the received data.

The processing device may be designed to store price or identification data of articles to which the various response devices are attached and to relate the identification codes of the identified response devices to them.

The system may include display means for generating a display associating price data with the articles to which the respective response devices are attached.

The system may further comprise printer means for producing a printout of the display.

According to a further aspect of the invention there is provided an interrogator for identifying a plurality of responders, comprising the interrogator having transmitter means for sending an interrogation signal to the responders, receiver means for receiving response signals from the responders, and processing means for identifying the responders based on the data in the response signal, wherein the interrogator can disable each responder, characterized in that the interrogator transmitter transmits at least two intermittent interrogation signals, there being an interval between successive interrogation signals which is shorter than a minimum period in which responders which have been disabled can automatically reset themselves.

According to a third aspect of the invention there is provided a method for identifying a plurality of responders, comprising the steps of: sending an interrogation signal to the responders, each responder receiving the interrogation sending a signal containing data in response, receiving the response signal from the responders, identifying the responders based on the data in the response signal, determining successful identification of a responder, locking the responder, and characterized in that at least two intermittent interrogation signals are sent, with an interval between successive interrogation signals which is less than a minimum period in which responders which have been locked can automatically reset themselves.

Short description of the drawings

Fig. 1 is a diagram showing the creation of a dead spot in an interrogation area as a result of a reflected signal;

Fig. 2 is a schematic diagram showing a first embodiment of the invention;

Fig. 3 is a diagram showing the effects of using different frequencies for the interrogation signal;

Fig. 4 is a schematic diagram of a second embodiment of the invention;

Fig. 5 is a schematic representation of a practical embodiment of the invention at a supermarket control;

Fig. 6 is a pictorial representation of an antenna arrangement of the system of Fig. 5;

Fig. 7 is a plan view of an antenna unit of the antenna arrangement according to Fig. 6;

Fig. 8 is a graph showing the radiation characteristic of an antenna element of the antenna unit of Fig. 7;

Fig. 9 is a schematic block diagram showing the overall electronic circuit structure of the system of Figs. 5 and 6;

Fig. 10 is a more detailed schematic block diagram of a quadrature receiver/amplifier of Fig. 9;

Fig. 11 is a wave diagram showing the waveforms at various points in Fig. 10;

Fig. 12 is a sample customer receipt printed by the system according to Figs. 5 and 6.

Description of the embodiments

Fig. 1 illustrates a problem that occurs in identification systems of the type mentioned when a reflective surface is located near the interrogator 10 and/or the interrogation area in which it is desired to locate responders. A primary interrogation signal 12 is transmitted directly from the antenna 14 of the interrogator 10 to the interrogation area, while a secondary interrogation signal 16 is reflected from the reflective surface. At certain distances from the interrogator, the direct and reflected signals 12 and 16 are half a wavelength out of phase, causing dead spots (zeros) in the electric field of the interrogation signal. This results in areas 20 of the interrogation area having a weak interrogation signal with insufficient RF energy to drive the transponders. As a result, certain transponders may not be located by the interrogator.

Fig. 2 schematically illustrates a first solution to the problem. In Fig. 2, the interrogator 10 comprises first and second antennas 22 and 24 spaced one-half wavelength apart and which can be selected by means of a switching device 26. Due to the different arrangement of the antennas, the dead spots or low power areas 20 occur at different locations. In use, the interrogator 10 is first connected to the antenna 22 and interrogates articles in the interrogation area, recording the identity codes received from the various transponders attached to the articles. The switching device 26 then connects the interrogator 10 to the antenna 24, repeating the process. The identification codes recorded during both interrogation processes are compared, discarding duplicate codes. In this way, all articles in the interrogation area can be identified, even though some of them are located in sections of the interrogation area which are in an RF dead center of one of the antennas 22 or 24.

The above system is suitable for identifying articles each of which has a transponder with a unique identification code. However, if a number of articles are provided with transponders all having the same identification code, it is not possible to count the number of articles accurately using the system of Fig. 2, since it is not possible to compare the results of the first and second interrogation processes in such a way that duplicate readings can be discarded.

The system described in South African Patent Application No. 92/0039, the contents of which are incorporated herein by reference comprises a number of identical transponders attached to articles of the same type to enable automatic inventory to be carried out. Each transponder, after successfully communicating its presence to the interrogator, is locked and remains in a locked state until the RF field induced by the interrogation signal is completely removed. It is obvious that a system having deep RF dead spots in its interrogation area is not suitable for use with this type of tag since individual tags may interpret the absence of RF power at a dead spot as a shutdown of the interrogator. As a result, a transponder which has been locked after successful identification could be turned back on if the dead spot position moved, thereby providing an additional signal and thus causing an incorrect count.

To overcome this problem, an interrogator is provided which intermittently transmits interrogation signals at at least two different frequencies. For example, frequencies of 750 MHz and 915 MHz may be used. These frequencies are chosen so that there is no location in the interrogation range where an RF dead spot is present at both frequencies, as shown in Fig. 3. Since the transponders are driven by rectifying the received RF energy from the interrogation signals, and since RF energy is present at every location in the interrogation range of at least one of the interrogation signals, the transponders remain constantly energized and can remember a "lockout command" received from the interrogator after a successful identification.

The interval between successive transmissions must be less than the minimum period within which locked response devices can automatically reset.

Since the responders transmit their identity codes either by changing the reflectivity of their receiving antenna or by re-radiating a percentage of the energy of the received an interrogation signal modulated with the identification code, this data is transmitted on both frequencies for those tags irradiated by both interrogation signals simultaneously, and only on one frequency for those responders located in the dead center of one or the other interrogation signal. Due to this, the interrogator can recognize the responders responding on one or both frequencies.

A system for implementing this embodiment of the invention is schematically shown in Figure 4. In this system, the interrogator includes an interrogator/controller unit 28, a first transmitter 30 with an associated antenna 32, and a second transmitter 34 with an associated antenna 36. Tags or transponders 38 are shown distributed in an interrogation area located adjacent to a reflective surface 40. Dead spots or areas of low RF field intensity 42 and 44, which are spaced apart and do not overlap, are schematically shown.

There are numerous practical applications of the invention. In one application, the system is used in a supermarket to automate the checkout process and eliminate the need for manual inquiry or entry of prices using a cash register. In another application, for example, the contents of a storeroom, warehouse or truck can be determined without unloading or unpacking. In another application, items such as books in a bookstore or library or CDs in a music store can be identified and counted by an automated inventory process.

It is understood that these examples are given only as examples and many other applications of the invention are possible.

A practical embodiment of the above-mentioned supermarket control according to the invention will now be described in more detail.

Fig. 5 shows an interrogator according to the invention which is installed at a supermarket checkpoint and which is designed so that the contents of a supermarket shopping trolley 46 which is pushed through an antenna unit 48 of the interrogator can be detected. The interrogator comprises a cash register or control unit 50 which has a large or small keyboard 52, a display device 54 and a receipt printer 56. The interrogator/control unit 50 is operated by a cashier or checkout assistant as in a conventional supermarket.

The antenna assembly 48 of the interrogator is shown pictorially in Fig. 6, where it can be seen that it comprises a frame of welded tubular sections which supports three separate antenna units 58, 60 and 62.

The frame supporting the antenna units is sized to allow the shopping cart 46 to slide under the upper antenna unit 60 and between the left and right side antenna units 58 and 62, which are oriented to define an interrogation area large enough to cover the interior of the shopping cart as it is pushed past the antenna units. The antennas of the various antenna units are polarized differently from one another to account for the fact that the articles in the interrogation area may be arbitrarily oriented so that their transponder antennas are also randomly polarized.

Inside the shopping cart are various items, including food items including bottles, boxes and other containers, as well as larger items that are not in a box or container, which can, however, be identified by a tag, sticker or label, for example.

Each article 64 in the shopping cart 46 has a transponder built into or attached to the article which has an identification code that uniquely identifies the type of article to which it is attached. Like articles are provided with transponders having identical codes. Some of the articles in the shopping cart may be identical and therefore have transponders having identical codes.

The three antenna units 58, 60 and 60 operate at different frequencies. The left side antenna unit 58 operates at 915 MHz, the right side antenna unit 62 operates at 910 MHz and the upper antenna unit 60 operates at 920 MHz.

Each antenna unit 58, 60, 62 comprises a transmit antenna and a receive antenna. The transmit and receive antennas are identical. Each antenna is a microstrip circuit assembly (see Fig. 7) comprising four square circuits 66 connected to one another. The transmit and receive antennas are polarized in the E plane and were formed on a Diclad GY870 printed circuit board material having a 3.2 mm thick copper coating and a substrate with a dielectric constant of 2.33 and a loss factor of 0.0012 when the prototype was installed. The antenna circuits 66 were 104 mm² in size and each circuit assembly was 406 mm² in size. Fig. 8 is an E-diagram of the radiation pattern of the microstrip switch array at 915 MHz, showing its relative directional characteristics.

Fig. 9 is an overall block diagram of the system's interrogator showing antenna units 58, 60 and 62 and their associated electronic circuits. The transmit antennas of each antenna unit 58, 60 and 62 are driven by respective transmitters 68, 70 and 72 operating at center frequencies of 910 MHz, 915 MHz and 920 MHz (i.e., 5 MHz apart). The transmitters 68, 70 and 72 are controlled by transmitter control signals generated by a microprocessor-based control unit 74 connected to a central computer system or, as in the present example, to the cash register 50. The interrogation signals transmitted by each transmitter comprise a carrier signal (at the corresponding operating frequency of the transmitter) modulated by signals addressing particular responders, particular groups or types of responders, or all responders.

The receiving antenna of each antenna unit 58, 60 and 62 is connected to a corresponding cavity resonant filter 76, 78, 80 tuned to the same frequency as the corresponding transmitter (i.e., 910 MHz, 915 MHz or 920 MHz). The output signals of the filters 76, 78 and 80 are fed to corresponding quadrature receivers and amplifiers 82, 84, 86 together with signals from the corresponding transmitter derived from the transmitter's local oscillator and a quadrature version of the local oscillator signal. The respective quadrature receiver/amplifiers produce data output signals which are fed to a combiner circuit 88 which combines the data signals in a synchronized manner and feeds a composite data signal to a phase-locked loop and a code extraction circuit 90 which reads out the codes contained in the received transponder signal and feeds them to the microprocessor 74.

The operation of the quadrature receiver/amplifiers 82, 84 and 86 is described in more detail below with reference to Fig. 10 which is a block diagram of a single quadrature receiver/amplifier and to Fig. 11 which is a wave diagram showing the waveforms present at various points in the circuit of Fig. 10.

The transmitters 68, 70 and 72, the cavity resonance filters 76, 78 and 80, the quadrature receivers/amplifiers 82, 84 and 86 and others associated RF components are housed in the housings of the respective antenna units 58, 60 and 62. The antenna units are connected to the combiner 88 and the microprocessor 74 in the housing of the cash register 50 by the cables 64. The cables carry data back and forth between the antenna units and the control and processing circuits of the interrogator and also supply power to the antenna units.

After being stimulated by the received interrogation signals, the transponders attached to the articles 64 in the shopping cart 46 begin to respond, sending their own identification codes back to the interrogator by modulating the received interrogator carrier frequency, as described in South African Patent Application No. 92/0039. Since each transponder is a relatively wide bandwidth device and has an antenna typically designed to receive signals from 800 MHz to 1 GHz, the transponders can respond to one or more of the signals transmitted by the respective antenna units at their different frequencies. The transponder transmitters must, of course, transmit at frequencies within the receive bandwidth of the transponders (in this case at frequencies between 800 MHz and 1 GHz).

The transponder circuitry is designed so that when the transponder transmits its identification code at the clock frequency of its on-board oscillator (typically 10 KHz), the effective input impedance of the transponder circuit is varied, thereby varying the cutoff and reflectivity of the transponder antenna accordingly. Therefore, a portion of the received interrogation signal is reflected back to the transponder antenna modulated with the transponder output signal. In this mode of operation, it is possible for the transponder's interrogation signal to be received by the transponder at one, two or all three different frequencies selected by the respective antenna units 58, 60 and 62, the transponder reflecting a modulated signal back to each antenna unit at the different, respective frequencies. It makes no difference to the operation of the transponder whether it is irradiated by one or more different frequencies, the reflected signals at the respective different frequencies not interfering with each other because of the relatively narrow bandwidth of the antenna units 58, 60 and 62 and their associated circuitry, and because the data modulation bandwidth of the interrogation signals is typically selected to be between 10 KHz and 100 KHz, which is substantially less than the spacing between the different interrogation frequencies.

A transponder response signal received by one of the antenna units is fed via the corresponding receive antenna and its associated cavity resonant filters to a mixer/filter circuit 92 where the received signal is mixed with a local oscillator signal received from the associated receiver to extract the modulation frequency band transponder response signal. The mixer/filter circuit 92 includes a low pass filter to reject the higher frequency products resulting from the different frequencies of the adjacent interrogator transmitters. The output of the mixer/filter circuit 92 is a signal A (see Fig. 11) which is fed to a high pass filter 94 where the code transitions in the transponder response signal are extracted by pseudo-differentiation. The response signal is shown in Fig. 11 at B.

The demodulated modulation frequency band transponder response signal A varies in strength and also contains inherent low frequency noise due to the Doppler shift of the interrogation signal carrier frequencies as objects are moved into the interrogation area. The high pass filter 94 filters out the low frequency noise, allowing only the relatively high frequency transitions of the code to pass through and efficiently amplifying the resulting "spikes". These transient "spikes" are further amplified by an amplifier circuit 96 resulting in the amplified signal C of Figure 11. The signal C is then passed through a full wave rectifier 98. The resulting full wave rectified signal is designated D1. The received transponder response signal is passed through an identical receiver circuit, but with its mixer/filter circuit 92' being supplied with a phase-shifted version of the local oscillator signal which is 90° out of phase with the local oscillator signal supplied to the mixer/filter circuit 92. The output of the dual receive circuit is a full wave rectified signal D2.

The outputs of full wave rectifier circuits 98 and 98' are added in a summing circuit 100 to produce a composite waveform E. When transitions or "spikes" occur together, the summed output is relatively large, whereas when only a weak signal occurs, the summed output is relatively small. The purpose of the dual receiver arrangement is to handle the situation where a received signal is not detected because the received signal is exactly in phase with the transmitter's local oscillator reference signal. By using an additional, out-of-phase local oscillator signal in a dual receiver channel, at least one of the signals D1 or D2 produces a strong output in response to a received signal.

The output of the summing circuit 100 is applied to an amplifier 102 which applies the amplified, combined signal to a noise limiting circuit 104 which is set to produce output clock pulses when it receives input pulses above a reference threshold. These clock pulses are applied to a D-type flip-flop 106 which produces an output signal F which is the received Manchester code format signal received by the transponder. The codes of the transponder response signal are designed to that the first bit of a transponder message is always a Manchester "1" which corresponds to the format of the codes regenerated by flip-flop 106.

In the waveform diagram of Figure 11, waveforms D1 and D2 correspond to signals received by a transponder which differ slightly in amplitude. When summed to produce a signal E, the "peaks" of signals D1 and D2 are added to obtain relatively strong signals. When the signals are sufficiently large in amplitude to exceed a threshold 108 of noise limiting circuit 104, a Manchester code output signal transition F is produced.

Each of the antenna units and their corresponding transmitter and receiver circuits operate in a similar manner so that each of the quadrature receiver/amplifiers 82, 84 and 86 can receive a response signal from a transponder using its own interrogation frequency and antenna polarizations.

In this regard, the situation may arise that articles located in the shopping cart 46 have transponders with antennas that are differently polarized as they are tossed around in the shopping cart in an uncontrolled manner. If articles are stacked in a truck or a warehouse, the articles might be stacked in a uniform manner, but the antennas of the transponders on the articles might be horizontally polarized, whereas a single interrogator antenna might be vertically polarized and therefore would not "see" the transponders. However, in the illustrated arrangement, the use of three different operating frequencies along with the differently polarized antennas ensures that the transponders in the shopping cart are generally simultaneously irradiated by at least two different interrogation frequencies, if not all three.

Metallic objects such as cans in the shopping cart may partially shield the contents of the shopping cart from one of the antenna units. In most cases, however, the other two antenna units will normally radiate the responders in question. If the polarization of one of these antenna units is incorrect for the responder in question, the remaining antenna unit should identify the responder. Of course, it is conceivable that the situation could arise where a responder is completely shielded by all three antenna units. However, this is unlikely in practice. In situations where it is of great importance to identify all items in an interrogation area, additional antenna units can be provided.

For example, in the example described above, another antenna unit could be provided below the shopping cart, either in addition to or instead of the upper antenna unit. The antenna arrangement could define a bay in which the shopping cart is temporarily "parked", rather than a "tunnel" through which the shopping cart is pushed. This would facilitate the placement of an additional antenna unit at the inner end of the bay.

The Manchester code data generated by each quadrature receiver/amplifier 82, 84 and 86 is fed to a combiner circuit 88 which includes a circuit which adds the three incoming waveforms in an analog manner to form a single combined response signal. The circuit is followed by a comparator and a single flip-flop to regenerate a single Manchester code as described above. The output of the combiner circuit 88 is therefore a Manchester code containing 64 bits of information and always beginning with a "1".

The output signal of the combiner circuit is fed to the phase-locked loop 90 and the microprocessor 74, which reads the information from the received code, as shown in the south African Patent Application No. 93/6267. The microprocessor reads the transponder identification code from the received signal, verifies that the code is a valid number by means of a parity check or cyclic redundancy check, and processes the number according to the relevant application.

When the microprocessor 74 determines that a transponder has been validly identified, the corresponding transmitters 68, 70 and 72 are instructed to modify their respective interrogation signals, for example by completely stopping the output signals or by reducing their output power by a predetermined amount at a certain time after successful receipt of the transponder response signal. This process is carried out in accordance with the system described in South African Patent Application No. 93/6267, the contents of which are incorporated herein by reference.

Transmissions from individual transponders are often "blocked" by overlapping transmissions from other transponders, so that the received signals fail one or another of the test/verification steps. However, if a transponder signal is received during a "silent" period when other transponders are not simultaneously transmitting, it is verified and the resulting data is fed to microprocessor 74 for identification and counting of the article to which the transponder is attached.

The system described above takes advantage of the fact that low-cost transponders of the type in question use components with a high tolerance, which allows good yields during manufacture. These transponders do not contain tuned circuits, but comprise a single integrated circuit which can be manufactured in a conventional integrated circuit manufacturing facility. The antenna of the transponder determines its frequency response characteristics and can be designed for a relatively wide bandwidth. These transponders can then be interrogated on several different frequencies, under Use of relatively narrow bandwidth interrogator transmitter/receiver antennas so that the responders modulate multiple interrogation signals when a response is sent.

When all of the responders in the shopping cart 46 have been successfully identified, which may typically take less than one second, the microprocessor 74 passes the data to the cash register 50, which produces a printout that may have the form of the sample printout shown in Figure 12, with the received responder codes associated with information from a price list. The type of each item in the shopping cart is displayed, as well as the price per item, the number of items, the subtotal and the total price of all of the items in the cart. The microprocessor 74 or cash register 50 itself may store the price data, which may be updated periodically, e.g., daily. Alternatively, the microprocessor 74 or cash register 50 may be connected online to a central computer that provides the updated price data in a consistent manner.

The information in the sample printout according to Fig. 12 can be displayed on the screen 54 of the cash register 50 and is reflected on a paper printout produced by the printer 56, which serves as a customer receipt. Payment by the customer can be made in the conventional way. However, the automatic generation of a receipt by the system described offers automatic invoicing for customers who have an account with the retailer in question.

Claims (40)

1. Identification system comprising an interrogation device and a plurality of response devices, - wherein the interrogation device comprises a transmitter device for sending an interrogation signal to the response devices, a receiver device for receiving response signals from the response devices and a processing device for identifying the response devices based on the data in the response signals, - each responder comprising receiving means, a code generator and transmitting means connected to the code generator, so that on receipt of the transmitted interrogation signal the responder sends a response signal containing data identifying the responder, - wherein the interrogator can disable each responder, characterized in that the interrogator sends at least two intermittent interrogation signals, there being an interval between successive interrogation signals which is shorter than a minimum period in which responders which have been disabled can automatically reset themselves. 2. Identification system according to claim 1, wherein the at least two interrogation signals each have different frequencies which are selected so that they fall within the reception bandwidth of the receiving devices of the response devices. 3. Identification system according to claim 2, wherein the at least two interrogation signals are signals with a relatively narrow bandwidth, the receiving devices of each responder have a relatively wide receiving bandwidth within which the respective different frequencies of the at least two interrogation signals fall, so that the responder responds to each individual or multiple interrogation signals. 4. Identification system according to claim 3, wherein each interrogation signal is modulated with data, the data modulation bandwidth of each interrogation signal being less than the distance between the respective different frequencies of the interrogation signals. 5. Identification system according to one of claims 1 to 4, wherein the transmitter means of the transponder comprises an antenna and means for modulating the reflectivity of the antenna so that the response signal of the transponder comprises one or more interrogation signal carriers modulated with the data identifying the transponder. 6. Identification system according to one of claims 1 to 5, wherein the transmitter device of the interrogation device comprises at least two spaced-apart transmitting antenna elements and the receiver device comprises at least two spaced-apart receiving antenna elements. 7. Identification system according to one of claims 1 to 5, wherein the transmitter device and the receiver device comprise at least two antenna units spaced apart from one another, wherein each antenna unit comprises a transmitting antenna element and an adjacent receiving antenna element. 8. Identification system according to one of claims 6 or 7, wherein each antenna element comprises a switching arrangement designed to operate at a frequency of between 800 MHz and 1 GHz. 9. Identification system according to one of claims 6 to 8, wherein at least two of the respective transmitting and receiving antenna elements are polarized differently from one another. 10. Identification system according to one of claims 1 to 9, wherein the transmitting device of the interrogation device comprises a transmitting antenna element, at least a first and a second transmitter for generating interrogation signals with different frequencies frequencies and switching devices for switching the output signals of the transmitters as an alternative to the transmitting antenna element. 11. Identification system according to one of claims 1 to 10, wherein the transmitter device and the receiver device of the interrogation device are mounted on or adjacent to a structure which defines an interrogation area through which the response devices to be identified can be passed. 12. Identification system according to claim 11, wherein the transmitter device and the receiver device of the interrogation device are supported by a frame which defines a passage through which transport of articles to which the respective response devices are attached can take place. 13. Identification system according to one of claims 1 to 12, wherein the at least two interrogation signals each have different frequencies which are selected such that there are no overlapping dead spots in the electrical fields of the interrogation signals within a predetermined distance of the transmitter device of the interrogation device. 14. An identification system according to any one of claims 1 to 13, comprising processing means for recording data received from each identified transponder and for relating the received data to stored data corresponding to the received data. 15. Identification system according to claim 14, wherein the processing means is adapted to store price or identification data of articles to which the various response devices are attached and to relate the identification codes of the identified response devices thereto. 16. Identification system according to claim 15, comprising display means for generating a display in which descriptions of the articles to which the respective responders are attached are associated with price data. 17. Identification system according to claim 16, comprising printer means for producing a printout of the indication. 18. An interrogation device for identifying a plurality of respondence devices comprising: - the interrogator, which has transmitter means for sending an interrogation signal to the responders, receiver means for receiving response signals from the responders and processing means for identifying the responders on the basis of the data in the response signal, - wherein the interrogator can disable each responder, characterized in that the interrogator transmitter transmits at least two intermittent interrogation signals, there being an interval between successive interrogation signals which is shorter than a minimum period in which responders which have been disabled can automatically reset themselves. 19. An interrogation device according to claim 18, wherein the at least two interrogation signals each have different frequencies. 20. An interrogator according to claim 19, wherein the at least two interrogation signals are relatively narrow bandwidth signals. 21. An interrogator according to claim 20, wherein each interrogation signal is modulated with data, the data modulation bandwidth of each interrogation signal being less than the spacing between the respective different frequencies of the interrogation signals. 22. Interrogation device according to one of claims 18 to 21, wherein the transmitter device of the interrogation device comprises at least two spaced-apart transmitting antenna elements and the receiver device comprises at least two spaced-apart receiving antenna elements. 23. An interrogation device according to any one of claims 18 to 21, wherein the transmitter device and the receiver device comprise at least two spaced apart antenna units, wherein each antenna unit comprises a transmitting antenna element and an adjacent receiving antenna element. 24. An interrogator according to any one of claims 22 to 23, wherein each antenna element comprises a switching arrangement designed to operate at a frequency of between 800 MHz and 1 GHz. 25. An interrogation device according to any one of claims 22 to 24, wherein at least two of the respective transmitting and receiving antenna elements are polarized differently from one another. 26. Interrogation device according to one of claims 18 to 25, wherein the transmitting device of the interrogation device comprises a transmitting antenna element, at least a first and a second transmitter for generating interrogation signals with different frequencies, and switching devices for switching the output signals of the transmitters alternatively to the transmitting antenna element. 27. An interrogator according to any one of claims 18 to 26, wherein the transmitter means and the receiver means of the interrogator are mounted on or adjacent to a structure that defines an interrogation area. 28. An interrogator according to claim 27, wherein the transmitter means and the receiver means of the interrogator means are supported by a frame which defines a passageway through which transport of articles can take place. 29. An interrogation device according to any one of claims 18 to 28, wherein the at least two interrogation signals each have different frequencies which are selected such that there are no overlapping dead spots in the electrical fields of the interrogation signals within a predetermined distance of the transmitter device of the interrogation device. 30. A method for identifying a plurality of responders comprising the following steps: - Sending a query signal to the responders, - each responder receiving the query sending a signal containing data in response, - Receiving the response signal from the response devices, - Identifying the responders based on the data in the response signal, - Determining successful identification of a responder, - locking the responder, and characterized in that at least two intermittent interrogation signals are sent, with an interval between successive interrogation signals which is shorter than a minimum period within which responders which have been locked can automatically reset themselves. 31. Method according to claim 30, wherein the respective different frequencies of the at least two interrogation signals are selected such that they fall within the reception bandwidth of the response devices. 32. The method of claim 31, wherein the at least two interrogation signals have a relatively narrow bandwidth and the response devices have a relatively wide receiving bandwidth within which the respective different frequencies of the at least two interrogation signals fall. 33. The method of claim 32, wherein each interrogation signal is modulated with data, the data modulation bandwidth of each interrogation signal being less than the spacing between the respective different frequencies of the interrogation signals. 34. A method according to any one of claims 30 to 33, wherein the response signal from a responder comprises one or more interrogation signal carriers modulated with data identifying the responder. 35. A method according to any one of claims 30 to 34, which comprises switching between interrogation signals. 36. Method according to one of claims 30 to 35, wherein the at least two interrogation signals each have different frequencies which are selected such that there are no overlapping dead spots in the electrical fields of the interrogation signals within a predetermined distance of the transmitter device of the interrogation device. 37. A method according to any one of claims 30 to 36, comprising recording data received from each identified transponder and correlating the received data to stored data corresponding to the received data. 38. A method according to claim 37, wherein the price or identification data of articles to which different response devices are attached are stored and the identification codes of the identified response devices are related to them. 39. The method of claim 38, comprising displaying descriptions of the items to which the respective response devices are attached and associating the items with pricing data. 40. The method of claim 39, comprising printing a hard copy of the display.